Firing 5.56 in a .223 chambered rifle

I own an FNC semi-auto that is stamped "CAL. 223 REM SPORTER". Now the Belgian military FNC is chambered for 5.56 NATO. I got a gonga deal on 2,000 rds of 5.56 many years ago and have been firing those thru the FNC ever since. I was always under the impression that the two rounds were identical.

Now I am reading that there is a *slight" difference in the chamber dimensions of .223 weapons and 5.56 weapons- and that higher than acceptable pressures can occur if 5.56 is fired in a weapon chambered for .223.

So what do you all say? Is this overreactive hype? or should I be concerned?

The .223 Remington is a sporting cartridge with the same external dimensions as the 5.56x45mm NATO military cartridge. It is loaded with a .224" diameter, jacketed bullet, with weights ranging from 40 up to 90 grains, though the most common loadings by far are 55 grains.

The primary difference between .223 Remington and 5.56 x 45 mm is that .223 is loaded to lower pressures and velocities compared to 5.56 mm. .223 Remington ammunition can be safely fired in a 5.56 mm chambered gun, but the reverse can be an unsafe combination. The additional pressure created by 5.56 mm ammo will frequently cause over-pressure problems such as difficult extraction, flowing brass, or popped primers, but in extreme cases, could damage or destroy the rifle. Chambers cut to .223 Remington specifications have a shorter leade (throat) area as well as slightly shorter headspace dimensions compared to 5.56 mm "military" chamber specs, which contributes to the pressure issues.

While the 5.56 mm and .223 cartridges are very similar, they are not identical. Military cases are made from thicker brass than commercial cases, which reduces the powder capacity (an important consideration for handloaders), and the NATO specification allows a higher chamber pressure. Test barrels made for 5.56mm NATO measure chamber pressure at the case mouth, as opposed to the SAAMI location. This difference accounts for upwards of 20,000+ psi difference in pressure measurements. That means that advertised pressure of 58,000 psi for 5.56mm NATO, is around 78,000 psi tested in .223 Rem test barrels (SAAMI .223 Rem Proof MAP is 78,500 psi so every 5.56mm round fired is a proof load, very dangerous). The 5.56 mm chambering, known as a NATO or mil-spec chambers, have a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 chambering, known as the "SAAMI chamber", is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber[2] or the Armalite chamber, which are designed to handle both 5.56 mm and .223 equally well.

Using commercial .223 cartridges in a 5.56-chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223-chambered gun due to the excessive leade. [3] Using 5.56 mil-spec cartridges (such as the M855) in a .223-chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and the SAAMI recommends against the practice.[4] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56 mm, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56 mm ammunition.

303tom, would you please edit your post. They are not the same round and someone searching via google with this same question might come across this thread and see your reply. They are similar but it's dangerous to shoot a 5.56 round in a .223 chambered firearm.

And before you get started on the thickness of the military case, yes they are that is to accommodate rough handling so you don`t have a bunch of dented up cases.

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The thickness is not to promote rough handling the military doesnt reload. Military brass is recycled or left on the battlefield. Recycled brass the military sends back is melted to make new cases. The thickness is to prevent case failure due to the higher pressure loading of the 5.56 cartridge.

Im not sure where you are getting your info. But the two are not the same at all. Ive personally seen two kabooms in the last decade from using nato surplus in civvy ar15 chambered for 223.

Military chambers have a long lead so they can use tracer ammo safely which has a greater bullet length than the same weight bullet in fmj format. Accuracy in military arms is not the main goal so a little extra bullet jump to hit the lands is not a big deal if the thing shoots 3 inches instead of 2.

Believe the (MYTHS) if you wish I know better, I been dealing with firearms & reloading for over 40 years, enough said !

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When some folks are given myths as facts then shown the real fact they cling to the myth cuz thats just the way its always been. It doesnt mean your stupid or wrong to just accept new info. It happens to me all the time. Taking it personally is the way to insanity.

Ive had my assumptions and misinformation on things challenged here and ive gone to the effort to look for myself. Its not bad or makes you bad to change your mind especially where safe use of firearms is concerned.

The two are close but they are very different. Much like 7.62x51 nato and 308 winchester. If you fire a 308 in a 7.62 chambering you stand a good chance of damaging your gun.

Believe the (MYTHS) if you wish I know better, I been dealing with firearms & reloading for over 40 years, enough said !

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I tell you what...you prove to me that all the folks online who recommend against firing a 5.56 in a chamber rated for .223 are idiots who don't know their butts from page one and I'll listen to you. Until then, maybe you should throttle back on the hubris a little bit.

Yep, the SAAMI myths are flying around today. US made 5.56mm cases are not thicker than US made .223 cases. The thickest US made cases are .223 Federal American Eagle and Gold Match. Some Brit 5.56mm military cases are thicker than any US made cases.

The thickest and heaviest cases are made by Lapua. For 45 years i've weighed the cases for my accuracy loads. Before getting on a board and posting stuff as fact, you should weigh some too.

The 5.56 mm NATO and .223 Remington cartridges and chamberings are similar but not identical. It is a myth that military 5.56mmx45 cases are made thicker and commercial .223 cases and military 5.56mm cases generally have the same case capacity.[17] However, the NATO specification allows a higher chamber pressure. NATO EPVAT test barrels made for 5.56 mm NATO measure chamber pressure at the case mouth, as opposed to the location used by the United States civil standards organization SAAMI. The piezoelectric sensors or transducers NATO and SAAMI use to conduct the actual pressure measurements also differ. This difference in measurement method accounts for upwards of 20,000 psi (140 MPa) difference in pressure measurements. This means the NATO EPVAT maximum service pressure of 430 MPa (62,000 psi) for 5.56 mm NATO, is reduced by SAAMI to 55,000 psi (380 MPa) for .223 Remington.[18] In contrast to SAAMI, the other main civil standards organization C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56 mm NATO.

The 5.56 mm NATO chambering, known as a NATO or mil-spec chamber, has a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 Remington chambering, known as SAAMI chamber, is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber (Rock River Arms)[19] or the ArmaLite chamber, which are designed to handle both 5.56 mm NATO and .223 Remington equally well. The dimensions and leade of the .223 Remington minimum C.I.P. chamber also differ from the 5.56 mm NATO chamber specification.

Using commercial .223 Remington cartridges in a 5.56 mm NATO chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223 Remington chambered gun due to the longer leade.[20] Using 5.56 mm NATO mil-spec cartridges (such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and SAAMI recommends against the practice.[21][22] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56 mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14 (marked ".223 cal"), but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56 mm NATO ammunition.[23]

It should also be noted that the upper receiver (to which the barrel with its chamber are attached) and the lower receiver are entirely separate parts in AR-15 style rifles. If the lower receiver has either .223 or 5.56 stamped on it, it does not guarantee the upper assembly is rated for the same caliber, because the upper and the lower receiver in the same rifle can, and frequently do, come from different manufacturers – particularly with rifles sold to civilians or second-hand rifles.

In more practical terms, as of 2010 most AR-15 parts suppliers engineer their complete upper assemblies (not to be confused with stripped uppers where the barrel is not included) to support both calibers in order to protect their customers from injuries and to protect their businesses from litigation following the said injuries.

The two are close but they are very different. Much like 7.62x51 nato and 308 winchester. If you fire a 308 in a 7.62 chambering you stand a good chance of damaging your gun.

Its the same principle as firing 38 in a 357.

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I have heard about the .223 vs 5.56 but the .308 vs 7.62 is news to me. I have a M-14 and have shot tons of the bulk battle pack ammo from CTD, but also shoot commercial .308 when that is all I can get my hands on for the weekend. Are you saying it will damage my M-14?

I have heard about the .223 vs 5.56 but the .308 vs 7.62 is news to me. I have a M-14 and have shot tons of the bulk battle pack ammo from CTD, but also shoot commercial .308 when that is all I can get my hands on for the weekend. Are you saying it will damage my M-14?

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I to shoot .308 in all my rifles chambered for the 7.62 NATO, my FR-7, my M-14, my 2A & 2A1, & you can`t shoot a .357 Mag. in a .38 SPL.

Using commercial .223 Remington cartridges in a 5.56 mm NATO chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223 Remington chambered gun due to the longer leade.[20] Using 5.56 mm NATO mil-spec cartridges(such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and SAAMI recommends against the practice.[21][22] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56 mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14 (marked ".223 cal"), but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56 mm NATO ammunition.[23]